1. Industrial Field
Etchants mainly composed of hydrogen fluoride alone or mixture of hydrogen fluoride and ammonium fluoride and containing additives for accelerating various chemical reactions have been commonly used not only for etching in the field of semiconductor production and other fields related thereto but also in the field of surface treatment of various materials such as metallic material, monocrystal material, optical material, etc. With increasing demand for the etchant, the importance of recovery techniques therefor has also increased a great deal in view of the need for environmental protection, particularly from the viewpoint of recycling.
In view of the foregoing, a primary object of the present invention is to establish a method for recovering fluorine from the used etchant in the form of purified calcium fluoride so that the recovered calcium fluoride may be effectively utilized again as a useful industrial resource in the field of production of hydrogen fluoride, refinement of metals, ceramic industry, etc.
2. Prior Art
Hitherto, a large number of techniques have been developed and proposed for treating a waste water containing fluorine with the use of calcium salt.
However, the reaction mechanism in the treatment of a solution containing ammonium fluoride utilizing the calcium salt has not been sufficiently studied in detail yet and, accordingly, there still remain several problems unsolved.
Looking briefly at past developments in the treatment of waste water containing fluorine, Japanese Laid-Open Patent Publication (unexamined) No. 51-19364 disclosed a technique for treating a waste water left after cleaning a stainless surface with hydrofluoric acid; Japanese Patent Publications (examined) Nos. 56-10120/57-39985/59-8438 disclosed a technique for eliminating fluorine and phosphorus from a waste water used in the process with phosphoric acid; and Japanese Patent Publications (examined) Nos. 56-144792/60-48191/61-25690/63-270595 disclosed a technique for treating a waste water left after semiconductor processing. In any of these prior arts, since it is intended to purify the waste water through the first and second treatments or separation of coexistant components or by combining separation steps, purification of calcium fluoride cannot be achieved.
U.S. Pat. Nos. 2,780,521 and 2,780,522 disclosed a technique in which calcium fluoride is produced with the use of calcium carbonate to separate silica contained in the filtrate for the purpose of recovering colloidal silica from the solution of fluorosilicic acid. By this technique, however, pure calcium fluoride recovery is limited to 92% CaF.sub.2 and 0.52% SiO.sub.2 per dry unit.
Japanese Laid-Open Patent Publication (unexamined) No. 50-142496 disclosed a method for adding calcium carbonate in two stages as a technique for purifying a waste water left after the process with wet phosphoric acid, and in which method excessive calcium carbonate was eliminated by cleaning with mineral acid.
Japanese Patent Publication (examined) No.57-47132 disclosed a technique for treating an exhaust gas containing fluorine comprising the steps of absorbing the exhaust gas in caustic potash; reacting potassium fluoride produced thereby by adding 1.0.+-.0.2 equivalent of potassium hydroxide; separating a precipitation layer mainly composed of calcium fluoride from a solution layer mainly composed of potassium hydroxide; neutralizing unreacted alkali such as Ca(OH).sub.2, KOH by adding an acid to the precipitation layer. In this technique, the reactivity of KF is 90% when 1.0 equivalent is added. Since there remains a lot of unreacted substances, neutralization with the use of hydrochloric acid is required, and moreover since concentration of residual fluorine ion is so high as to be 900 ppm, the concentration is reduced to 200 ppm by adding excessive equivalent of 1.2 equivalent.
Japanese Laid-Open Patent Publication (unexamined) No. 58-46355 disclosed a method of adding slaked lime in two stages as a technique for treating waste liquid containing ammonium fluoride. This method comprises a first stage of precipitating a high grade calcium fluoride for separation by adding slaked lime to fluorine in such a manner that the added amount of the former is a little smaller than that of the latter; and a second stage of precipitating a low grade calcium fluoride for separation by adding 2 equivalents of slaked lime to the residual fluorine; then returning the obtained low grade calcium fluoride to the first step, while distilling the residual solution to recover ammonia.
The mentioned method of adding slaked lime is not always appropriate for the solution containing hydrogen fluoride alone or a mixture of hydrogen fluoride and ammonium fluoride to be treated in the present invention. This is because fluorosilicic acid ion present in the etchant reacts with the slaked lime, whereby most of the silica is combined with the calcium fluoride obtained eventually, resulting in inhibition of high purification.
As mentioned so far, the prior art cannot solve the problems or meet the object of the invention in the manner described below.